In modern semiconductor fabrication, optical lithography is used to pattern layout features onto a semiconductor wafer. Current step-and-repeat optical lithography systems and step-scan-and-repeat optical lithography systems use illumination sources with wavelengths of 365 nanometers (nm) (Hg I-line), 248 nm (KrF B-X excimer laser), and 193 nm (ArF B-X excimer laser). Historically, it has been possible to use illumination wavelengths larger than the layout feature sizes to be patterned. However, as layout feature sizes continue to decrease without a corresponding decrease in available illumination wavelength, the difference between the smallest wavelength and smallest layout feature size has become too large for successful lithographic rendering of the features. For example, lithographers are having great difficulty patterning 65 nm layout feature sizes with the 193 nm illumination source. Moreover, layout feature sizes are expected to continue to decrease from 65 nm to 45 nm, and on to 32 nm.
Lithographers use the following formula from Lord Rayleigh to estimate the optical system capability: resolution (half-pitch)=k1 λ/NA, where k1 is a fitting variable roughly corresponding to a process window, λ is the illumination wavelength, and NA is the numerical aperture of the lithographic system. When the wavelength λ was larger than the half-pitch, typical values for k1 were over 0.50. Because the feature size has been decreasing by a factor of 0.7 for each technology node, the value of k1 has been steadily decreasing for each technology node, while the wavelength λ has been constant, and the NA has been increasing by only 1.1 to 1.2 per technology node step. Additionally, for a NA greater than about 0.93, immersion systems are needed in which water replaces air as the medium between the final lens and the photoresist on the wafer. These immersion systems are expected to support a NA up to about 1.35, with no clear, cost-effective solution thereafter.
In view of the foregoing, a solution is needed to enable patterning of nano-scale feature sizes without having to further decrease illumination wavelength λ and/or further increase numerical aperture NA.